(a) Field of the Invention
The present invention relates to a negative active material for a rechargeable lithium battery and a method of preparing the same and, more particularly, to a negative active material which exhibits high capacity and good charge and discharge efficiency.
(b) Description of the Related Art
Metallic lithium was initially used as the negative active material in rechargeable lithium batteries. However, metallic lithium causes an abrupt capacity loss and deposits to form a dendric layer which induces short circuit between the negative and positive active material and a reduction in the cycle life of the battery.
The problem of lithium reactivity toward the electrolyte has been addressed by replacing metallic lithium with carbon-based materials. With the use of carbon-based active materials, the potential safety problem present in metallic lithium-based batteries can be prevented while achieving a relatively higher energy density as well as a reasonable shelf life.
There are two main types of carbon-based materials used crystalline graphite and amorphous carbon. Crystalline graphite includes artificial graphite and natural graphite. Typical examples of artificial graphite may include mesocarbon fiber (MCF) and mesocarbonmicro beads (MCMB). There has been introduced somewhat recently artificial graphite, which is doped with a different element from carbon to produce an inexpensive negative active material exhibiting good electrochemical performance.
Mesophasecarbon microbeads and mesophase fiber are easily produced, but are expensive. Also, because of a stabilizing step that must be performed, mesophasecarbon microbeads and mesophase fiber have substantially lower discharge capacities than natural graphite. Natural graphite on the other hand has a high discharge capacity, but a higher irreversible capacity than mesophasecarbon microbeads or mesophase fiber. Also, because natural graphite is plate shaped, discharge properties are extremely deteriorated.
To overcome such problems, various studies has been conducted on shapeless and low coke-based artificial graphite in which elements such as boron are used as a doping material to enhance the overall performance of the battery.
Japanese Patent Laid-open No. Hei 3-165453 (Central Devices Co.) discloses a method in which a BCN(H) type negative active material is produced using a CVD (Chemical Vapor Deposition) procedure.
Japanese Patent Laid-open No. Hei 3-245458 (Sony) discloses a method in which organic materials are carbonized by adding 0.15 to 2.5 wt % of B compounds using an autoclave procedure to prepare a negative active material.
Japanese Patent Laid-open No. Hei 5-266880 (Matsushita) discloses a in which B, B2O3 or H3BO3 is added to petroleum pitch and the mixture is heat-treated at 2000xc2x0 C. for 20 hours to prepare a negative active material.
Japanese Patent Laid-open No. Hei 9-63584 (Petoca) and Hei 9-63585 (Petoca) disclose methods in which after pulverizing mesophase fiber, B compounds are added to the fiber and the mixture is heat-treated.
However, in all of the above methods, a negative active materials results that is deficient in at least one important aspect including capacity, charge and discharge efficiency, current density, and cycle life.
It is an object of the present invention to provide a negative active material for a rechargeable lithium battery which exhibits high capacity and good charge and discharge efficiency.
It is another object to provide the negative active material for a rechargeable lithium battery which exhibits high current density and good cycle life.
It is another object to provide a method of preparing the negative active material for a rechargeable lithium battery.
These and other objects may be achieved by a negative active material for a rechargeable lithium battery including a crystalline carbon core and a semi-crystalline carbon shell formed on the core. The semi-crystalline carbon shell includes at least one element or a compound of at least two different types of elements, and has a turbostratic or half onion-sheath. The compound includes only element. The element serves graphitization catalyst and causes a change in a structure of surrounding carbon.
The negative active material includes 0.01 to 20 wt % of metal boride.
In order to achieve these objects and others, the present invention provides a method of preparing a negative active material for a rechargeable lithium battery. In this method, amorphous carbon or crystalline carbon is mixed with at least one element, or compound thereof. The compound of the element includes an A-B compound and an A-C compound. A and B are respectively at least one element and C is not an element. Thus, the A-B compound refers to a compound including only the element and the A-C compound refers to a compound including another component in addition to the element. The element serves graphitization catalyst and causes a change in a structure of surrounding carbon. Following the mixing step, the mixture is graphitized at 2000 to 3200xc2x0 C.